A known image forming apparatus requires an input of predetermined authentication information at the time of starting printing. Such an image forming apparatus is configured to allow an external authentication device to be attached thereto and detached therefrom. The external authentication device is configured to acquire authentication information. When the external authentication device is attached to the image forming apparatus and authentication information acquired by the external authentication device matches prestored information, the image forming apparatus permits an information processing device to transmit print information to the image forming apparatus and then performs printing.